1. Field of the Invention
The present invention relates to a photoconductive device. The present invention also relates to a terahertz wave generation apparatus and a terahertz wave detection apparatus that each use the photoconductive device.
2. Description of the Related Art
Terahertz waves are electromagnetic waves having any components of frequency bands within a range from 0.03 THz to 30 THz. In the terahertz wave band, there exist many characteristic absorptions that depend on the structures and states of various materials including biological molecules. Utilizing such a characteristic, nondestructive inspection technologies for analyzing and identifying materials have been developed. Terahertz waves are also expected to be applied to safe imaging technologies replacing X-ray technology and high-speed communications technologies.
One of the important factors for making these technologies practically usable is the advance in terahertz wave generation and detection technologies. As a terahertz wave generation technology, a photoconductive device technology has been disclosed (see Japanese Patent Laid-Open No. 2006-86227). The photoconductive device described in this document uses a photoconductive layer of indium gallium arsenide (InGaAs) that can efficiently absorb 1.5 μm band excitation light. Further, photoconductive devices with increased resistance have been disclosed, such as a device in which iron (Fe) ions have been injected in the photoconductive layer thereof and a device in which a portion of the photoconductive layer thereof other than a portion irradiated with excitation light has been removed. These devices, in the case of photoconductive devices appropriate for 1.5 μm excitation light, may be able to generate high-intensity terahertz waves.
Hitherto, the thickness of a photoconductive layer has been made larger than an optical absorption length for the excitation light wavelength in order to increase the absorption amount of the excitation light. However, this makes the thickness of the photoconductive layer larger than that of a depletion layer, and thereby sometimes causes a decrease in the resistance of the photoconductive layer. When the resistance of a photoconductive device is low, an applied voltage causes a high current to flow, which is likely to damage the photoconductive device. This causes a voltage applied to a photoconductive device to be lowered. Since terahertz waves are generated when photo-excited carriers are accelerated by an electric field within a photoconductive layer, the intensity of terahertz wave radiation is likely to be small if a voltage which can be applied to the photoconductive device is low as described above. It is hard to claim that such problems have been solved even by the technology disclosed in Japanese Patent Laid-Open No. 2006-86227.